1. Field of the Invention
The present invention relates to a magnification varying optical device (an image reading device) for giving magnified information of an object to a light receiving plane, and more precisely it relates to an optical device for varying a magnification by changing a distance between an object and an image thereof.
2. Description of Related Art
A magnification varying optical device (image reading device) is usually used in a copying machine having a variable power, an image scanner, a facsimile, or a digital copying machine, or the like, in which a photoelectric element, such as a CCD is used as a light receiving element.
There are mainly two types of magnification varying devices, one of which has a zoom lens system for keeping constant a distance between an object and an image, the other type having an imaging lens having a fixed focus, in which the magnification is varied by changing the distance between the object and the image, as well as the position of the imaging lens. The former type, in which the zooming lens system is used, has advantages in that a high precision variation of power (magnification) can be effected and that the distance between the object and the image is constant. Since the former type device includes a large and expensive zoom lens, it can not be applied to an optical instrument or system which must be compact and inexpensive.
On the other hand, the latter type of magnification varying device, in which an imaging lens having a fixed focus is used, is less expensive, but has a drawback in that when the fixed focus lens is moved relative to an object plane and an image plane to vary the magnification, the imaging efficiency deteriorates, since the fixed focus lens is usually designed so that a clearest image is formed at a specific magnification.